Biaryl compounds such as biphenyl, dimethyl biphenyl, and other various substituted biphenyl derivatives can be functionalized and converted to monomers for production of various high performance polymeric compounds such as polyesters or polyamides. Various biaryl compounds can be obtained by homocoupling of the same or heterocoupling of different monoaryl compounds. Coupling of monoaryls is an efficient technique for production of a variety of biaryls because monoaryls are readily available and inexpensive starting materials.
Coupling of aryl halides is one of the known methods for production of biaryls. This method is disclosed, for example, in "Applied Homogeneous Catalysis with Organometallic Compounds", Vol.2, B. Cornis and W. A. Herman, 1996, VCH, and is known as the Heck reaction. While effective in production of biaryls, this method suffers from the need for costly purification processes and recycling of its side products, i.e., metal halides.
Another method, also disclosed in the Cornis reference and known as the Suzuki reaction, is the coupling of aryl boron. This method, however, includes the strenuous step of preparation of boron intermediates. As with the aryl halide coupling, this method also suffers from costly recycling and disposal problems.
R. Van Helden et al., Rec. Triv. chini., 1965, 84,1263 reported oxidative coupling of benzene and substituted benzenes using palladium chloride and sodium acetate in acetic acid solution. The reported reaction took place at high temperatures and metallic palladium is precipitated in the reaction system.
In the Van Helden et al. process the palladium compound is a reactant rather than a catalyst. As such, it is used in these processes in stoichiometric amounts in the dimerization reaction. Since palladium is a very rare metal and its compounds are extremely expensive, use of palladium compounds in stoichiometric amounts, rather than catalytic amounts, makes these processes commercially impractical and the final products expensive.
U.S. Pat. No. 4,008,266 to Intille discloses coupling of monoaryl compounds in the presence of a mercuric oxyanion and a Pd (II) compound. The reaction takes place at elevated temperatures and high pressures. A minimum oxygen pressure of 200 psi is required in order for the coupling reaction to proceed as a catalytic reaction.
Oxidative coupling of monoaryls in the presence of Pd (II) catalysts and trifluoroacetic acid, with or without addition of Cu (II) co-catalysts, is reported in J. Chem. Soc., Perkin I, 1974, 1289. However, the reaction time, which may be up to 14 days, is too long for large scale, commercially feasible production. Moreover, the reaction product contains a mixture of biaryl isomers such as 3,3', 3,4'- or 4,4'-isomers, and a high amount of di-aromatic compounds.
Coupling of monoaryl compounds using catalytic amounts of palladium (II) acetate is disclosed in U.S. Pat. Nos. 3,895,055 to Itatani et al. and 4,294,976 to Itatani et al. While this technique uses only catalytic amounts of palladium (II) compounds when air is used as an oxidant, the reaction takes place at elevated temperatures of about 150.degree. C. and an oxygen partial pressure of at least 5 kg/cm.sup.2. For example, while the toluene coupling rate at 150.degree. C. and air pressure (50% oxygen) of 750 psi is about 18.5 mmol/hour, the estimated toluene coupling rate at room temperature is only about 0.046 mmol/hour based on activation energy data for this reaction.
The coupling of monoaryl compounds in the presence of a Pd (II) acetate catalyst is disclosed in U.S. Pat. Nos. 3,963,787 and 4,164,518, both to Ichikawa et al. An increase of catalytic activity is reported for use of organic carboxylates, halogen, and oxyhalogen compounds of zirconium in conjunction with the palladium (II) catalyst. To achieve any considerable yield of biaryl compounds, the reaction takes place at high temperatures of 100.degree. C. and above and at high oxygen pressure.
The present invention provides a catalytic system obtained by admixing a palladium (II) compound and a strong acid or mixture of acids for coupling of monoaryl compounds. The method of coupling monoaryl compounds in the presence of catalytic amounts of the catalytic system may be used to selectively produce isomers, such as 4, 4'-isomers, for the production of desired polymers. The reaction conditions can be easily adjusted to yield different isomers of biaryl compounds in different proportions, and the reactions proceed to high yields at room temperatures within relatively short reaction times.